1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device including a circuit formed of a thin film transistor (hereinafter referred to as a TFT) using a crystalline semiconductor film formed on a substrate having an insulating surface. Particularly, the present invention relates to a liquid crystal display device in which a pixel portion and a driving circuit provided at its periphery are provided on the same substrate and an electrical instrument (called electronic apparatus as well) using the liquid crystal display device as a display portion.
2. Description of the Related Art
With the rapid development of an information-oriented society, information appliances including a personal computer (PC) rapidly become popular for not only a business enterprise but also an individual. From the viewpoint of space savings of portable information equipment or a display of the PC, a liquid crystal display device (liquid crystal display) is regarded as being promising from the early days. However, there has been a problem that a manufacturing process of the liquid crystal display device is complicated and its yield is low, and accordingly, manufacturing costs are high.
Besides, in recent years, from a problem of a field effect mobility, technical development of a thin film transistor (hereinafter referred to as a TFT) using a polycrystalline semiconductor film, for example, silicon as semiconductor (hereinafter referred to as a polysilicon film or a crystalline silicon film), which has a polycrystalline state obtained by crystallizing a noncrystalline semiconductor film (hereinafter referred to as an amorphous silicon film) formed on an insulating surface provided on a substrate (for example, a glass substrate, a quartz substrate, a stainless steel substrate, etc.), has been rapidly advanced. Especially, a polycrystalline silicon film prepared by carrying out a heating treatment for crystallization at a low temperature (600° C. or lower) is called a low temperature polysilicon film.
In recent years, researches have been carried out to construct a semiconductor circuit by forming TFTs on a glass substrate or the like. As an electric device such a semiconductor circuit, an electro-optic device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided over the same substrate. Further, the development of a system-on-panel having a built-in logic circuit, such as a memory circuit or a clock generating circuit, has also been advanced.
Since such a driver circuit or a logic circuit requires performing a high speed operation, it is unsuitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, in the present circumstances, a TFT including a crystalline silicon film (polysilicon film) as an active layer has become mainstream.
Then, research and development has been actively carried out as to a process, a so-called low temperature process, for forming a large area crystalline silicon film on a substrate having low heat resistance as compared with a quartz substrate, such as a glass substrate.
As a method of preparing a low temperature polysilicon film, a laser annealing method, an ion doping method, or the like is mainly used. As a method of obtaining a high quality low temperature polysilicon film, a technique using a metal element as a catalytic element for facilitating crystallization is disclosed in Japanese Patent Application Laid-open No. Hei 7-183540, etc. As the metal element, nickel (Ni), palladium (Pd), lead (Pb), tin (Sn) or the like is used. The catalytic element is added to a semiconductor (silicon) film by a method, such as a solution coating method, a sputtering method, an ion implantation method, an evaporation method, or a plasma treatment method, and a heating treatment for crystallization is carried out. However, there has been a problem that although such a treatment can be carried out at a low temperature, a treatment time is long.
The present inventors et al. disclose a technique for obtaining a crystalline silicon film on a glass substrate in Japanese Patent Application Laid-open No. Hei 7-130652. In the technique of the invention, a catalytic element for facilitating crystallization is added to an amorphous silicon film, and a heating treatment is carried out to crystallize the amorphous silicon film.
By this crystallization technique, it became possible to lower the crystallization temperature of the amorphous silicon film by 50 to 100°C., and to shorten a time required for crystallization by a factor of ⅕ to 1/10. As a result, it became possible to form a large area crystallized silicon film even on a glass substrate having low heat resistance. It is experimentally confirmed that the crystalline silicon film obtained by such a low temperature process has excellent crystallinity.
Besides, an environmental problem becomes more serious, and it is emergently required to take energy-saving measures with respect to electric appliances at worldwide level. Then, in order to achieve such an object as improvement of efficiency of a manufacturing process for mass production of liquid crystal cells or reduction of manufacturing costs, enlargement of a substrate in the manufacturing process is required, and technical development for obtaining a plurality of TFT substrates from a large glass substrate has been advanced.
Incidentally, in the present specification, the liquid crystal cell indicates a display device in a state where a liquid crystal is interposed between a substrate on which pixel TFTs are formed and a counter substrate.
The present applicant discloses, in Japanese Patent Application Laid-open No. Hei 7-130652, a method of fabricating a crystalline semiconductor film having high crystallinity by adding a metal element (hereinafter referred to as a catalytic element) having a function of facilitating crystallization to an amorphous semiconductor film in a crystallization step and by carrying out a heating treatment.
However, the method of the above invention is a heating treatment using a furnace, and it takes a rather long time, for example, 1 to 14 hours to carry out the heating treatment and to form the crystalline semiconductor film.
In the manufacturing process for actually mass-producing semiconductor devices, shortening of a treatment time is an important problem.
Besides, as another technique for improving the efficiency of the manufacturing process, the establishment of a technique of manufacturing a plurality of liquid crystal cells, for example, six liquid crystal cells each having a size of 12.1 inches from one large glass substrate, for example, a substrate of 550 mm×650 mm has also been advanced. In future, a technique and a manufacturing apparatus for manufacturing more liquid crystal cells from a larger glass substrate are required to be introduced. With the enlargement of a mother member (glass substrate) before treatment, an apparatus used for the manufacturing process is naturally required to be enlarged, and a furnace for carrying out the heating treatment has a problem on the enlargement of an installation area, and needs energy for uniformly and sufficiently heating the large furnace for treating the large substrate as set forth above, and there has been a problem that the energy becomes enormous electric power consumption.
Then, in view of the efficiency of manufacture and the improvement of productivity, it is conceivable that an RTA (Rapid Thermal Anneal) method is suitable as a heating method. However, the RTA method is a method in which a heating treatment of a high temperature and a short time is carried out for the purpose of suppressing the diffusion of an impurity in a semiconductor layer, and in a heating treatment step of a semiconductor film requiring the diffusion of an element, such as a crystallization step using a catalytic element or a gettering step, there is a possibility that a glass substrate is distorted before a desired effect is obtained. For example, it is confirmed that in the gettering step in a furnace, the glass substrate is curved and deformed by its own weight by merely carrying out a treatment at 800° C. for 60 seconds.